Most of conventional constant speed control methods for a vehicle use an electronic control unit to control an opening degree of a throttle valve for maintaining a constant speed of the vehicle. For example, Taiwanese patent issued No. I312325, which was filed by the applicant, discloses a constant speed device for a scooter and a constant speed cruise program applying thereto. With reference to FIG. 8, when a scooter driver presses a constant speed on/off switch, a controller detects all kinds of data. When determining the speed of the scooter is lower than a constant speed, the controller adjusts a throttle valve and a fuel nozzle to increase air flow and fuel sent to a combustion chamber. On the other hand, when determining the speed of the scooter is higher than the constant speed, the controller reversely adjusts the throttle valve and the fuel nozzle to decrease the air flow and fuel sent to the combustion chamber.
However, when being putting into practical use, the foregoing constant speed device for a scooter and the constant speed cruise program applying thereto still have the following problems: when the constant speed device for a scooter adjusts the opening degree of the throttle valve and the fuel nozzle to change the air flow and the fuel sent into the combustion chamber for maintaining the speed of the scooter, a rotation speed of an engine of the scooter will fluctuate that the fuel in the combustion chamber will not achieve the best combustion efficiency due to the sudden variation of the rotation speed of the engine. As a result, even though the conventional constant speed device for a scooter can achieve an objective of maintaining constant speed cruise, the fuel consumption is simultaneously raised.
Hence, it is necessary to provide a constant speed control method and a device thereof to overcome the problems existing in the conventional technology.